1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a fabrication method thereof. Particularly, the present invention relates to the liquid crystal display in which liquid crystal is sealed between two panels by using an instilling method and the fabrication thereof.
2. Description of the Related Art
A liquid crystal display panel of a conventional liquid crystal display is described with reference to FIG. 104. FIG. 104 shows a part of an upper surface of an active matrix-type liquid crystal display panel using a TFT (thin film transistor) as a switching element viewed from a color filter substrate side. As shown in FIG. 104, on a liquid crystal display panel 1100, a plurality of pixel areas 1114 arranged in a matrix shape are formed on an array substrate 1116 side, and a TFT 1112 is formed in each pixel area 1114. A display area 1110 is structured by the plurality of the pixel areas 1114. It will be noted that although a detailed illustration is omitted, a gate electrode of a TFT 1112 in each pixel area 1114 is connected to a gate wiring and a drain electrode is connected to a data wiring respectively. Further, a source electrode of the TFT 1112 is connected to a pixel electrode formed in the pixel area 1114. A plurality of the data wirings and the gate wirings are connected to a terminal portion 1102 formed in the external periphery of the array substrate 1116 so that the plurality of the data wirings and the gate wirings are connected to a driving circuit (not shown in the diagram) provided externally.
A color filter (CF) substrate 1104 formed smaller than the array substrate 1116 by approximately the area of the terminal portion 1102 is provided facing the array substrate 1116 while sealing liquid crystal at a predetermined cell gap. On the CF substrate 1104, a common electrode (not shown in the diagram) is formed and at the same time, BM (black matrix: shading film) 1080, 1180 and the like using color filters (shown by letters R (red), G (green) and B(blue) in the diagram), a Cr (chrome) film or the like are formed. Since a BM 1118 demarcates the plurality of the pixel area 1114 in the display area 1110 and earns contrast, the BM 1118 is used for preventing a light leakage from occurring by shading the TFT 1112. Further, a BM picture-frame portion 1108 is provided for shading undesired light from outside the display area 1110.
The array substrate 1116 and the CF substrate 1104 are attached by a sealing material 1106 made of photo-curing type resin.
Incidentally, a fabrication process of the liquid crystal display is roughly classified into an array process in which a wiring pattern, a switching element (in a case of active matrix type) and the like are formed, a cell process in which an alignment layer treatment is performed, spacers are arranged and liquid crystal is sealed between opposing glass substrates, and a module process in which an installation of a driver IC, attachment of a back light and the like are performed. In a liquid crystal injection process performed in the cell process among the above processes, a method (vacuum injection method) is used, for example, in which the array substrate 1116 forming the TFT 1112 and the opposite color filter substrate (opposite substrate) 1104 are attached with a use of the sealing material 1106, the sealing material is cured, then liquid crystal and the substrates are placed in a vacuum chamber, an injection opening opened in the sealing material is immersed in liquid crystal, the inside of the chamber is returned to the atmospheric pressure, thereby sealing the liquid crystal between the substrates.
On the other hand, in recent years, an instilling method is drawing attention, in which, for example, a constant amount of liquid crystal is dropped on the substrate surface inside the frame of the sealing material 1106 formed inside a frame shape in the periphery of the array substrate, the array substrate 1116 and the CF substrate 1104 are attached in a vacuum and sealing of liquid crystal is performed.
A fabrication process of the liquid crystal display panel according to the instilling method is briefly described with reference to FIGS. 108a through 108c. First, as shown in FIG. 108a, for example, liquid crystal 1206 is dropped from a liquid crystal instilling equipment which is not shown in the diagram at a plurality of positions on an array substrate 1204 forming switching elements such as TFT and the like. Next, a common electrode and a color filter are formed in a display area, and an opposite substrate 1200 coated with a UV sealing material 1202 to be cured by ultraviolet (UV) irradiation in the external periphery of a display area is aligned and attached to the array substrate 1204. This process is performed in a vacuum. Then, when the attached substrates are returned to the atmospheric pressure as shown in FIG. 108b, the liquid crystal 1206 between the attached array substrate 1204 and the opposite substrate 1200 is spread due to the atmospheric pressure. Next, as shown in FIG. 108c, while a UV light source 1208 travels in a travel direction 1211 along the area the sealing material 1202 is coated, UV light is irradiated to the sealing material 1202 and the sealing material 1202 is cured.
In comparison to the vacuum injection method widely used for panel fabrication in the past, this instilling method has possibilities to reduce costs of fabricating a panel and to improve mass productivity owing to first, substantial reduction in the amount of liquid crystal to be used and second, reduction in time to inject liquid crystal and the like, therefore application of this instilling method is strongly desired in the panel fabrication process.
For example, in the Japanese Lain-open Patent Application No. 63-179323, a method is recorded in which an accurately measured required amount of liquid crystal is mounted on a substrate surface inside a sealing material provided on one substrate, the opposite other substrate is overlaid so that the substrate contacts with an upper surface of the sealing material before the liquid crystal spreads on the first substrate surface and reaches an end face of the peripheral sealing material, then both substrates are pressed in a decompressed environment, and the sealing material is cured.
However, in the above method, although basic processes of instillation to follow is shown, a specific description relative to a fabrication technology is insufficient and in reality, technical problems remain in practical application of the process. The instilling process, in comparison to the liquid crystal injection process performed in the past, enables to simply fabricate a liquid crystal panel at low cost but at the same time has technical difficulties as shown below resulting in delay in adopting the instilling method in a fabrication method of a liquid crystal display.
(1) Curing Defects of Sealing Material:
If uncured components of the sealing materials 1106 and 1202 make contact with liquid crystal for a long period of time or are exposed in high temperature while contacting with liquid crystal, the liquid crystal is contaminated. Therefore, photo-curing-type resin which is rapidly cured by ultraviolet light irradiation is used for the sealing materials 1106 and 1202 when the instilling method is used.
Incidentally, the width of a picture-frame portion in the periphery of a panel is becoming narrow due to recent enlargement of the liquid crystal panel 1100 and the like. Therefore, the sealing material 1106 formed in a frame shape in the periphery of a substrate is formed in many cases in the very close proximity to the end of an external periphery of the BM picture-frame portion shown in FIG. 104. Accordingly, when the array substrate 1116 and the CF substrate 1104 are pressed and the area (hatched area in FIG. 104) where the sealing material 1106 and the BM picture-frame portion 1108 make contact is generated, the area of the sealing material 1106 where the BM picture-frame portion 1108 contacts is shaded and not irradiated by light resulting in generating a curing defect area in the said area.
(2) Seal Peeling:
FIGS. 105a and 105b show liquid crystal instillations in the cell process of a liquid crystal panel in the past. FIG. 105a shows a state when liquid crystal (shown by a mark ◯) 1144 is dropped in equal intervals (in this example, matrix shape of three rows and four columns) in a similar shape to the frame shape of the sealing material 1106 on an upper surface of the array substrate inside the sealing material 1106. With respect to a dropping position of each liquid crystal 1144, the distance to a dropping position of an adjacent liquid crystal 1144 has a relation, as shown in the diagram, which is d2=d4=d6=d8>d1=d3=d5=d7. FIG. 105b shows a state in which the liquid crystal 1144 spreads after the array substrate and the CF substrate are attached. As shown in FIG. 105b, while the sealing material 1106 is formed in a rectangular frame shape, fluid drops of the dropped liquid crystal 1144 spread in a circular shape 1146 on the substrate surface. In a conventional dropping method, since fluid drops interfere with one another, approximately 20 minutes of time is required to sufficiently lessen a space 1145 and complete spreading liquid crystal.
Thus, in the conventional method, a long period of time is required to spread liquid crystal to corner portions of the sealing material 1106 and a waiting period for curing the sealing material is long. Accordingly, due to a difference in pressures between the inside and outside of both substrates, possibilities of occurring peeling of corner portions of the sealing material during the waiting period and generating liquid crystal leakage are high.
(3) Substrate Deformation and Display Irregularities:
Substrate holding in liquid crystal instillation in the conventional process is performed by using vacuum chucks, electrostatic chucks or a mechanical retainer. In the substrate holding by vacuum chucks, a substrate is mounted on an attracting surface on a parallel surface plate and is fixed by vacuum-absorbing a back surface of a substrate. By this holding method, for example, an array substrate is held and an adequate amount of liquid crystal is dropped on an array substrate surface inside the frame shape of a sealing material by a dispenser and the like. Then, a CF substrate is positioned in the vacuum environment and entered into a process to be attached with an array substrate. However, since vacuum chucks do not function when a degree of vacuum increases to a certain point when the substrates are held by vacuum chucks, the degree of vacuum at the time of attaching substrates can not be sufficiently increased. Therefore, sufficient pressure for attaching both substrates can not be coated and evenly attaching both substrates is difficult.
Further, in a mechanical holding, since stress is applied only to the holding side portion of a substrate, deformations such as a curvature, deflection and the like occur in a substrate, and both substrates can not be held in parallel when attaching substrates after liquid crystal instillation. If the attachment is performed when both substrates are deformed, a displacement becomes large and problems of reduction in opening ratio of each electrode and light leakage from a shaded portion occur.
FIGS. 106a and 106b are diagrams describing substrate attachment by electrostatic chucks. FIG. 106a shows a plan view of an electrostatically attracted glass substrate 700 of the array substrate 1116 in two-piece structure as an example. FIG. 106b shows a cross section cut by a line A-A in FIG. 106a when the array substrate 1116 and the CF substrate 1104 are to be attached.
As shown in FIGS. 106a and 106b, the areas to become two pieces of array substrate 1116 on the glass substrate 700 are electrically isolated from each other. Electrostatic chucks for electrically attracting the glass substrate 700 has four electrodes 740, 750, 760 and 770 on a parallel surface plate. The electrodes 740 and 750 among the four electrodes 740 through 770 structure positive electrodes and the electrodes 760 and 770 structure negative electrodes. One surface of the array substrate 1116 is electrostatically attracted by the positive electrode 740 and the negative electrode 760 and the other surface of the array substrate 1116 is electrostatically attracted by the positive electrode 750 and the negative 770. Space 680 is provided in a boundary between the positive electrode 740 and the negative electrode 760 and in a boundary between the positive electrode 750 and the negative electrode 770. Although an illustration by a plan view is omitted, the electrostatic chucks on a glass substrate 720 forming the CF substrate 1104 has a similar structure to the electrostatic chucks attracting the glass substrate 700.
By mounting the glass substrate forming a conductive film on the electrostatic chucks in such structure, applying voltage between the electrode and the conductive film and generating the coulomb's force between the glass and the conductive film, the glass substrate can be attracted. In the case of FIGS. 106a and 106b, the conductive film on the glass substrate 700 includes the pixel electrodes, gate wirings, data wirings and the like formed on the array substrate 1116 area. Further, the conductive film on the glass substrate 720 forming the CF substrate area includes the common electrode and the like.
In order to attach substrates while holding the glass substrates 700 and 720 by such electrostatic chucks, the positive poles 740 and 750 are contacted to one of the two substantially equally divided areas of the array substrate 1116, the negative poles 760 and 770 are contacted to the remaining area, a predetermined voltage is applied between the positive and negative poles and the glass substrate 700 is electrostatically attracted. At this time, as shown in FIG. 106b, a surface corresponding to the positive poles 740 and 750 in the array substrate 1116 area of the glass substrate 700 is charged with negative (−) electricity and a surface corresponding to the negative poles 760 and 770 are charged with positive (+) electricity. Thus, on the conductive film of the array substrate 1116 corresponding to the air gap 680 of the boundary between the positive and negative poles, a boundary between a positive electric charge and a negative electric charge is formed.
Incidentally, an alignment film is formed on an upper portion of the conductive film of the array substrate 1116 and liquid crystal is dropped on the alignment film by instillation. Therefore, if the array substrate 1116 area is electrostatically attracted according to the above method, impure ion in liquid crystal is selectively attracted on the alignment film at both sides of the boundary dividing the surface of the array substrate 1116 area into substantially two equal parts. Accordingly, the above method has a problem of generating display irregularities in which when a formed liquid crystal panel is displayed, the brightness in the two surfaces sandwiching the said boundary varies.
Further, when the glass substrate 700 forming the array substrate 1116 and the glass substrate 720 forming the CF substrate 1104 are attached while being held by electrostatic attraction, if voltage in reversed polarity of positive or negative is applied on the opposing surfaces of both glass substrates 700 and 720 as shown in FIG. 106b, the coulomb's force is operated to each of the opposing substrates resulting in reduction of the substrate holding strength due to electrostatic attraction. Thus, possibilities of causing a substrate deformation or contacting the substrates with each other and causing electrostatic destruction exist.
Furthermore, a method in which substrates are held by electrostatic chucks of which the substrate holding strength is not affected by the degree of vacuum also has a problem in which a glow discharge occurs in the course of decompressing the atmospheric pressure for attaching substrates and may generate damage to a circuit or a TFT element on a substrate. Also, a phenomenon may occur in which an operation of the electrostatic chucks becomes unstable due to the air remained between the electrostatic chucks and the substrates, and the substrates break off from the electrostatic chucks in the course of the substrate attachment process.
(4) Variations in Cell Gap:
In order to evenly spread liquid crystal inside both substrates in the instillation process, liquid crystal is required to be dropped at multiple points on a substrate surface by dispenser or the like. However, since the amount of liquid crystal to be dropped per one substrate surface is minute, when dropping positions are scattered into multiple points, an extremely small amount of liquid crystal must be accurately dropped. Nevertheless, the amount of liquid crystal to be dropped varies due to variations in viscosity or volume of liquid crystal affected by changes in the environment such as temperature variations at the time of instillation or variations in quality of a dispenser. As a result, variations in cell gap between both substrates occur.
FIGS. 107a to 107c are cross sections cut vertical to a liquid crystal panel surface and shows an example of variations in cell gap. FIG. 107a shows a state in which a desired cell gap is obtained by an ideal liquid crystal instillation. In FIGS. 107a to 107c, the array substrate 1116 and the CF substrate 1104 are attached by the sealing material 1106 and a predetermined cell gap is secured by beads 1150 as spacers. However, if the amount of dropped liquid crystal increases, as shown in FIG. 107b, the sealing material 1106 can not be pressed to an intended gap due to excessive liquid crystal resulting in a problem in which display irregularities occur in the peripheral portion of a panel (periphery of picture-frame portion). When the amount of dropped liquid crystal is further increased, as shown in FIG. 107c, a phenomenon in which a center portion of a panel is expanded due to the sealing material 1106 causing a press defect occurs resulting in display irregularities on a whole surface.
(5) Degradation of Liquid Crystal:
Further, in a liquid crystal display fabricated by using the instilling method, a problem is generated in which display irregularities occur at the edge of a seal where a sealing material and liquid crystal contact. One of the causes is described with reference to FIG. 109. FIG. 109 shows a partial cross section of the end portion of a liquid crystal display panel. An array substrate 1200 and an opposite substrate 1204 face each other through the aid of a sealing material 1202. A pixel electrode and a bus line (in FIG. 109, these are collectively referred by a code 1212) are formed on the array substrate 1200 surface facing the opposite substrate 1204, an alignment film 1214 is formed on the surface 1212, a common electrode and a color filter (in FIG. 109, these are collectively referred by a code 1216) are formed on the opposite substrate 1204 surface facing the array substrate 1200, and an alignment film 1218 is formed on the surface 1216. A predetermined cell gap is kept and the liquid crystal 1206 is sealed between the opposing electrodes. As shown in the diagram, the liquid crystal 1206 at the end portion of a panel contacts with the sealing material 1202.
If UV irradiation is performed toward the sealing material 1202 for curing the sealing material in such a structure, UV light is slightly dispersed and a liquid crystal 1220 in a hatched area shown in the diagram adjacent to the sealing material 1202 is also irradiated. However, usually, if a liquid crystal material is irradiated by UV light, characteristics of liquid crystal are degraded, and specifically, resistivity tends to be reduced and high voltage retention ratio required in TFT-LCD and the like can not be kept. Therefore, operating voltage of a liquid crystal cell is different in comparison with a portion which is not irradiated by UV, display irregularities at half-tone display become prominent.
Further, since an area where the sealing material 1202 before UV irradiation and the liquid crystal 1206 make contact is large in an instilling method, the possibility of contaminating a liquid crystal material due to uncured sealing material is high. In order to suppress this liquid crystal contamination, a UV sealing material is required to be rapidly cured by instantly performing UV irradiation. However, there is a problem in which if a UV light high in strength is irradiated in order to reduce irradiation time, damage caused by the light leakage to the liquid crystal material also becomes large.
As described above, photo-curing resin or heat-curing resin is used for a sealing material in the instilling method. As preceding technologies relative to photo-curing a sealing material, a technique in which ultraviolet light is irradiated through a mask having a predetermined pattern transmitting light to attached substrates (Japanese Laid-open Patent Application No. 09-61829), a technique in which an upper and lower substrates are arranged facing each other so that a shaded portion is not overlapped with a position a seal is arranged (Japanese Laid-open Patent Application No. 09-90383), a technique in which a panel is pressed by a pressure difference between the pressure at the time of attachment of substrates and the atmospheric pressure or the pressure in a vacuum chamber after the attachment (Japanese Laid-open Patent Application No. 10-26763) and the like are known.
However, even if these techniques are used, the photo-curing process in the instilling method holds problems described below.
First, photo-degradation of liquid crystal can be cited. Although ultraviolet-light-curing resin is used for photo-curing resin because of the preservation ability and the adhesive strength as previously described, when ultraviolet light is irradiated to liquid crystal, photolysis reaction makes progress and an ion impurity is generated. This ion impurity causes display defects such as irregularities due to a reduction in voltage retention ratio or in image persistence. For this reason, a use of a mask having a predetermined pattern transmitting light as disclosed in the above document (Japanese Laid-open Patent Application No. 09-61829) is conceivable. However, this method of using a mask has a problem in which since a mask is required for each seal pattern and the number of processes is increased by a mask alignment process, the goal of the instilling method of liquid crystal such as reducing a fabrication cost of a panel and improving mass productivity may be rather prevented than accomplished.
Secondly, enlargement of an outside dimension of a panel can be cited. Usually a terminal made of many metal films is formed in a non-display area on the array substrate side. In order to arrange an upper and lower substrates facing each other so that a shading portion of the substrates do not overlap with a position a sealing material is arranged as described in the above document (Japanese Laid-open Patent Application No. 09-90383), essentially, a seal is required to be formed outside the picture frame of a black matrix, thereby resulting in enlargement of an outside dimension of a panel.
Thirdly, there is a problem of displacement. Since curing of a seal is instantly performed in photo-curing, the stress due to a waviness and curvature which are natural characteristics of a substrate tend to stay. If a heat treatment is performed in this state, the stress is released and a displacement of a substrate occurs.
Fourthly, there is a problem of press defect. In instillation, a whole substrate is pressurized by a pressure difference between the pressure at the time of attaching substrates and the atmospheric pressure or the pressure in a vacuum chamber after the attachment as described in the above document (Japanese Laid-open Patent Application No. 10-26763) to spread liquid crystal. Immediately after pressurization, since liquid crystal does not yet reach a sealing material, the sealing material is instantly pushed and pressed to the thickness of a spacer inserted between substrates. However, since the inside of the panel is thicker than a predetermined thickness, the sealing material is subsequently pushed back. Although the thickness of the panel gradually approaches the predetermined thickness and the sealing material is again pressed to the thickness of a spacer by extending shelf time, liquid crystal is contaminated from uncured sealing material in the time the liquid crystal is left. Therefore, as a matter of fact, curing is required to be performed in the least amount of time. Due to this balance, sufficient shelf time can not be taken and insufficient shelf time becomes a cause of generating press defect.
In the above vacuum injection method or instilling method, in order to cure a sealing material in a short period of time, photo-curing resin or photo plus heat-curing resin is used for a seal. However, in the instilling method, there is a possibility in which a sealing material contacts with liquid crystal when the sealing material is uncured. If a sealing material component elutes into liquid crystal or ultraviolet light is irradiated to adjacent liquid crystal when a sealing material is cured and liquid crystal is resolved by photolysis, the voltage retention ratio of liquid crystal at the edge of a seal is reduced, thereby occurring display irregularities.
In order to deal with this problem, for example, in the Japanese Laid-open Patent Application No. 06-194615, a liquid crystal display in which a column-shape spacer is arranged outside the pixel area on either one of a pair of substrates and a frame-shape spacer (frame-shape structure) is arranged along the fringe periphery of the said substrate is disclosed. These spacers are simultaneously formed in a photolithography process and are used to fabricate a liquid crystal panel using an instilling method.
FIG. 110a shows a part of an upper surface of a conventional active-matrix type liquid crystal panel 1100 different from the one using a TFT as a switching element shown in FIG. 104 viewed from a CF (color filter) substrate side. FIG. 110b shows a partial cross section cut at a line A-A of FIG. 110a. A plurality of pixel areas 1114 arranged in a matrix shape are formed on an array substrate 1116 side of the liquid crystal display panel 1100 and a TFT (not shown in the diagram) is formed in each pixel area 1114. A picture display area 1110 is formed by a plurality of pixel areas 1114.
A CF substrate 1104 is formed smaller than the array substrate 1116 by approximately the width of a terminal portion 1102 and arranged facing the array substrate 1116 while sealing a liquid crystal at a predetermined cell gap. The array substrate 1116 and the CF substrate 1104 are attached by a main seal 1106 made of photo-curing type resin. A width 1106′ shown by double dotted lines indicates the width of the main seal 1106 at the time of coating. A frame-shape structure 1111 separating the main seal 1106 and the liquid crystal 22 is formed in the area between the main seal 1106 and the display area 1110. The liquid crystal 22 is sealed in the area surrounded by the frame-shape structure 1111 between the array substrate 1116 and the CF substrate 1104.
A common electrode (not shown in the diagram) and color filters (indicated by letters R (red), G (green), B (blue) in the diagram) are provided on the CF substrate 1104. A BM picture frame 1108 and a BM deciding the space between pixel areas are also formed on the CF substrate 1104. An external peripheral end of the frame-shape structure 1111 is arranged inside an external peripheral end of the BM picture frame viewed from a direction vertical to the surface of the substrate 1116. Therefore, a peripheral end portion inside the main seal 1106 overlaps with a peripheral end portion outside the BM picture frame 1108 and an area 1107 is formed. Thus, UV light is shaded by the BM picture frame 1108 and a curing defect of the main seal 1106 is generated in the area 1107.
Further, as shown in FIG. 111, if the frame-shape spacer 1111 equivalent to a cell gap alone is provided in the fringe periphery of the CF substrate 1104 when liquid crystal more than the amount to fill the frame-shape spacer 1111 at instillation is dropped, excess liquid crystal flows over the frame-shape spacer 1111, uncured sealing material 1106 and the liquid crystal 22 make contact, thereby dispersing contaminant. Furthermore, as shown in FIG. 112, if a cell gap is thick, the liquid crystal 22 easily flows over the frame-shape spacer 1111 before the liquid crystal 22 is completely spread. FIG. 112 shows a state in which a surface of the array substrate 1116 is viewed from the CF substrate 1104 side. When the liquid crystal 22 is dropped at a plurality of liquid crystal dropping points by using a liquid crystal instilling method, and the substrates 1116 and 1104 are attached, a boundary 1123 of the liquid crystal 22 at the time of attachment is gradually spread. Before the liquid crystal 22 is completely spread, an uninjected portion of liquid crystal 1121 is formed, and even if there is no excess liquid crystal, since the cell gap is higher than the height of the frame-shape spacer 1111, the liquid crystal boundary 1123 flows over the frame-shape spacer 1111 and, for example, at a position 1122, contacts with uncured main seal 1106. Also, as shown in FIG. 113, since the atmospheric pressure evenly operates on the whole substrate surface when the substrates are returned in an atmosphere after attachment, the center of the substrate becomes depressed with respect to the main seal 1116 in which the resistance is larger. As a result, the frame-shape spacer 1111 is lifted up and the liquid crystal 22 contacts with the main seal 1106.
In addition to the problems described above, the conventional instilling method further holds problems identified below.
(6) Seal Peeling Due to a Curing Defect:
A black matrix (BM: shading film) is usually formed in the fringe periphery portion (picture frame) of a liquid crystal display substrate. Unless a frame-shape spacer is strategically arranged, when a sealing material is spread after substrates are attached, a part of the sealing material overlaps with the end of a BM picture frame and UV light is obstructed, thereby resulting in curing defects. Since adhesive strength is weak in the portion of a curing defect, external stress is concentrated and peeling of a sealing material is induced. If a position of a sealing material is sufficiently apart from the end of a BM picture frame, such defects do not occur. However, by so doing, the picture-frame area is enlarged and the glass substrate surface can not be efficiently utilized.
(7) Excess Liquid Crystal Flowing Over a Frame-shape Spacer:
If a column-shape spacer equivalent to a cell gap alone is provided in a fringe periphery of a substrate, when liquid crystal more than the amount to fill the frame-shape spacer is dropped at the time of instillation, an excess liquid crystal flows over the frame-shape spacer, uncured sealing material and liquid crystal contact, thereby dispersing contaminant. Further, when there are variations in a dropping amount from a dropping dispenser even if the dropping of liquid crystal is controlled, or when liquid crystal reaches the frame-shape spacer before liquid crystal fully fills inside the frame, since a cell gap is thick before liquid crystal is completely spread, liquid crystal easily flows over the frame-shape spacer.
(8) Irregularities Due to a Dropping Mark:
A liquid crystal display fabricated by an instilling method has a problem in which a “dropping mark” in the area liquid crystal is dropped is seen as an irregularity. FIGS. 114a through 114c show an example of the “dropping mark”. FIG. 114c shows a liquid crystal instillation in a state in which a dropped liquid crystal 136 is adhered on an alignment film 134 on a substrate 132. In display irregularities due to “dropping marks”, there are irregularities 130 as shown in FIG. 114a in which boundaries of the dropped areas are visible and surface irregularities 131 as shown in FIG. 114b in which the brightness of the whole dropped areas is different from the brightness of the periphery. After dropped liquid crystal and an alignment film contacts, an instilled panel is positioned and attached, and then liquid crystal is spread in a vacuum.
The cause of the “dropping mark” is considered to be a contact between liquid crystal and an alignment film in the atmospheric pressure. Further, a degree of the “dropping mark” is found to be different depending on a liquid crystal material for dropping and a material for an alignment film. If a liquid crystal material has strong polarity and a material for liquid crystal and a material used for an alignment film material are inferior in electrical characteristics (i.e. low voltage retention ratio, high ion density, large residual DC voltage), the “dropping mark” tends to be more visibly generated. Specifically, although in a liquid crystal panel in which alignment control of liquid crystal of MVA-(multi-domain vertical alignment) mode can be realized, N-type (negative dielectric anisotropy: Δ∈<0) liquid crystal material and a vertical alignment film are required, material selection for these materials is limited in comparison with P-type liquid crystal material and a horizontal alignment film, and there are only few materials among the existing materials which satisfy requirements of electrical characteristics. Therefore, liquid crystal even more reliable is required to be used for a liquid crystal material which contacts with an alignment film in the atmospheric pressure and a different fabrication method from before is required.
(9) Other Problems:
Furthermore, an instilling method has a problem in which administration to prevent substrates failed at instillation due to troubles in the process and substrates failed to create a cell gap adjacent to a main seal from advancing to subsequent processes is difficult. Specifically, since the birefringence of liquid crystal viewed from the front surface of a panel when voltage is not applies is equal to 0 in an MVA-mode liquid crystal panel, a liquid crystal layer is seen as the same as an air layer and grasping a state of liquid crystal instillation with certainty is difficult. Therefore, easily and steadily performing an inspection of display irregularities of a liquid crystal panel fabricated according to an instilling method is desired.
Also, in order to reduce contamination of liquid crystal due to a contact between liquid crystal and uncured sealing material, a use of a sealing material of high viscosity can be considered. However, a gap is difficult to create by a sealing material of high viscosity and a cell gap at the edge of a seal becomes thicker then a cell gap at the center of a display, thereby resulting in generating a problem of display irregularities.
Further, upon performing an instilling method, due to variations in the environment during the period up to when a sealing material is cured by UV irradiation after attaching substrates in a vacuum and subsequently returning the attached substrates in an atmosphere, changes in a condition of substrates at the time of UV irradiation, and a lack of stability in positioning substrates when a gap is created or the like, a displacement in attaching or a displacement from substrate distortion is generated, or a gap defect is generated, thereby resulting in a problem in which producing a stable product is difficult.